Alzheimer's disease (“AD”) is a devastating neurodegenerative disease that affects millions of elderly patients worldwide and is the most common cause of nursing home admittance. AD is characterized clinically by progressive loss of memory, orientation, cognitive function, judgement and emotional stability. With increasing age, the risk of developing AD increases exponentially, so that by age 85, some 20% to 40% of the population is affected. Memory and cognitive function deteriorate rapidly within the first five years after diagnosis of mild to moderate impairment, and death due to disease complications is an inevitable outcome. Definitive diagnosis of AD can only be made post-mortem, based on histopathological examination of brain tissue from the patient.
Two histological hallmarks of AD are the occurrence of neurofibrillar tangles of hyperphosphorylated tau protein and of proteinaceous amyloid plaques, both within the cerebral cortex of AD patients. The amyloid plaques are composed mainly of a peptide of 37 to 43 amino acids designated beta-amyloid, also referred to as beta-amyloid, amyloid beta or Abeta. It is now clear that the Abeta peptide is derived from a type 1 integral membrane protein, termed beta amyloid precursor protein (also referred to as APP) through two sequential proteolytic events. First, the APP is hydrolyzed at a site N-terminal of the transmembrane alpha helix by a specific proteolytic enzyme referred to as beta-secretase (the membrane-bound aspartyl protease BACE1). The soluble N-terminal product of this cleavage event diffuses away from the membrane, leaving behind the membrane-associated C-terminal cleavage product, referred to as C99. The protein C99 is then further hydrolyzed within the transmembrane alpha helix by a specific proteolytic enzyme referred to as gamma-secretase. This second cleavage event liberates the Abeta peptide and leaves a membrane-associated “stub.” The Abeta peptide thus generated is secreted from the cell into the extracellular matrix where it eventually forms the amyloid plaques associated with AD.
Despite intensive research during the last 100 years, prognosis of AD patients now is still quite the same as that of patients a century ago, since there is still no real cure available. There are two types of drugs approved by the U.S. Food and Drug Administration and used in clinics today to treat AD: Acetylcholinesterase (AchE) inhibitors and Memantine. There is ample evidence in the art that the amyloid beta peptide, the main component of the amyloid plaques that are specific to the AD etiology, has a key role in the development of AD disease (Hardy et al. 2002; Golde et al. 2006). Therefore, one of the most favorite strategies to lower Aβ is to diminish its production by γ- and β-secretase inhibitors. One strategy was the development of gamma-secretase inhibitors, however, such inhibitors often result in serious side effects since gamma-secretase is involved in the proteolytic processing of at least 30 proteins (De Strooper et al. 2003). Yet another attractive strategy is the development of β-secretase (BACE1) inhibitors, as BACE1 knock-out mice are viable and have no obvious pathological phenotype (e.g., Roberds et al. 2001; Ohno et al. 2004; Ohno et al. 2006).
BACE1, also named memapsin2 and Asp2, is a 501 amino acids type I membrane-bound aspartyl protease, and it shares significant structural features with eukaryotic aspartic proteases of the pepsin family (e.g., Hussain et al. 1999; Lin et al. 2000). Like other aspartic proteases, BACE1 has an N-terminal signal peptide (residues 1-21) and a pro-peptide (residues 22-45). The 21 amino acids signal peptide translocates the protease into the ER where the signal peptide is cleaved off and from where BACE1 is then directed to the cell surface. After its passage through the trans-Golgi network (TGN), part of BACE1 is targeted to the cell surface from where it is internalized into early endosomal compartments. BACE1 then either enters a direct recycling route to the cell surface or is targeted to late endosomal vesicles destined for the lysosomes or for the TGN. At the TGN, it might be retransported to the cell membrane. Given its long half-life and fast recycling rate, mature BACE1 may cycle multiple times between cell surface, endosomal system and TGN during the course of its lifespan (e.g., Huse et al. 2000; Wahle et al. 2005). BACE1-mediated cleavage of APP at the O-site occurs in early endosomes, where the acidic environment is optimal for its enzymatic activity. However, when APP containing the so-called Swedish mutation was used as cellular substrate, β-cleavage preferentially occurred in ER and TGN (Thinakaran et al. 1996).
Although BACE1 has become an established prime drug target for AD therapy, the development of effective inhibitor drugs for BACE1 remains quite challenging. Numerous efforts have been contributed to the rational design of small-molecular inhibitor drugs for BACE1, however, the progress has been challenged due to the large and unaccommodating nature of the BACE1 active site, and the need to develop a blood brain barrier (BBB) penetrating drug with high potency and high selectivity against other aspartic proteases. So, there is a need for alternative approaches targeted at BACE1 as potential therapies for AD.